Certain clinical orthopaedic procedures often involve the insertion of a bone substitute material or a device in to a bone void or defect. The defect may be created by disease, surgically created or may have resulted from traumatic injury to the bone. A bone void or defect in, for example, a long bone IM canal may be prepared or created to accept an implantable device through either debridement, reaming out the contents of the canal or simply by the removal of an intra-medullary nail or rod. The purpose of the bone substitute material or a similar material may be to regenerate bone, to provide structural support or to be a carrier for the delivery of a therapeutic agent to the bone void.
Current practice is to use a bone substitute material that can be dispensed or inserted into the bone void by a number of means. For example, a material having the consistency of a paste can be inserted or injected in to the void or defect using a syringe or similar device. Granules, beads or pellets can be digitally inserted one at a time or they may be loaded into a delivery device for subsequent dispensation into the void. It is important not to occlude the entrance to the bone void during material or device insertion as blocking a vent path whilst inserting the material or device into a contained void will result in increasing pressurisation of the bone void. This increase in pressurisation within the bone void may result in embolization, such as a fat embolism of the void contents in to the blood stream of the patient with disastrous consequences. Fat embolism syndrome (FES) is caused by fat droplets which are then found within the peripheral and lung microcirculation1. FES is a serious complication and can result in the death of the patient.
The pathologic significance of FES was first noted in 1862 by Zenker; however, the first person considered to diagnose fat embolism was Von Bergmann in 1873, who reported on his findings concerning this condition1, 4.
As patients are an aging population it means that the number of orthopaedic operations involving bone voids and more specifically the IM canal are increasing, thus increasing the risk of embolization or FES which in turn means an increase in perioperative cardiorespiratory emergencies1.
Patients suffering from FES may be asymptomatic for a period of 12 to 48 hours before the clinical manifestations of the syndrome; these manifestations include but are not limited to tachycardia, petechial rash, elevated temperature (usually in excess of 38.3° C.), hypoxemia and also neurological symptoms6.
The mortality rate for patients suffering from FES is from 5-15%3; however proper treatment of the patient through ensuring good arterial oxygenation, the restriction of fluids and the use of diuretics which assist in minimising the accumulation of fluid in the patient's lungs will assist in recovery.
A technique often employed by the treating surgeon in, for example, long bones or vertebral bodies to mitigate this problem is to provide a second hole known as a venting hole. In long bones this is usually done by drilling a secondary opening through the cortex into the medullary canal. This hole is usually placed distal to the hole which the surgeon created for the placement of the bone substitute material or device and provides an exit path which enables escape of the void contents ahead of the advancing bone substitute material.
This technique, however, has a number of problems. The bone substitute material which the surgeon injects or digitally packs into the long bone may escape through this venting hole. This therefore means that an insufficient amount of bone substitute material may remain within the void or IM canal while the escaped material may cause injury to the adjacent soft tissues2. Another issue found with this technique of drilling a venting hole through the cortical bone is that it can increase the risk of fracture at the site of the drilled hole.
Therefore, the problem to be solved is how to effectively and safely introduce an implantable device comprising a bone substitute material into a bone void, particularly a contained bone void such as the IM canal, whilst ensuring that the bone void does not become excessively pressurised.